Re: Medium Format Training

[T3D John Bercovitz <bercov@xxxxxxxxxxx>]

John Vala writes:

> Several years ago I bought a programmable calculator from Radio 
> Shack and put one of Ferwerda's formulas into it for calculating
> Stereo Base using Near-point, Far-point, lens focal length and 
> maximum parallax.  It goes   Baseline = (parallax X Near-point 
> X Far-point)/(Focal Length X (Far-point - Near-point))  I'm 
> wondering if this is the formula you mentioned in the Excel 
> Spreadsheet (which I haven't gotten to yet)?

When you download maofd.txt from the ftp/web site, you'll see that 
the formula found on the Excel spreadsheet is 

             an*af 
b0  =  d* [ ------- ( 1/f  - 1/a) ]
             af-an 

As you can see, Ferwerda's formula is a degenerate (and  
approximate) form of the one true formula.  8-)  "a" is the distance 
at which the camera is focussed.  So it doesn't matter if you use 
the exact formula or Ferwerda's approximation if you're not 
shooting something that is close relative to the focal length of 
the lens.

John B


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